1. Field of the Invention
This invention pertains generally to nuclear magnetic resonance spectroscopy, and more particularly to an apparatus and method for NMR spectroscopy by spatially and temporally remote signal detection or optical detection.
2. Description of the Background Art
Nuclear magnetic resonance (NMR) has developed into a very versatile analytical tool for the study of molecular structures and surface features. However, NMR is a relatively insensitive detection method compared to others since the NMR signal depends on the population difference between two spin states. A number of approaches have been taken to increase the spatial, temporal and spectral resolution of NMR devices. One approach to increasing sensitivity is increasing magnetic field strengths since NMR sensitivity increases as the 7/4th power of the strength of the magnetic field.
Another approach has been to improve the rf receiver coil size, geometry and component materials. It has been shown that the sensitivity of a detector coil is inversely proportional to the diameter of the coil. High temperature superconducting materials and cryogenically cooled detector coils have also improved the sensitivity of NMR devices.
One significant improvement in sensitivity was the discovery that NMR and MRI signals could be enhanced through the use of hyperpolarized Noble gases. Xenon and other Noble gases that are members of the zero group of the periodic table of elements, exhibit NMR characteristics that are highly sensitive to the chemical environment surrounding the atoms. The characteristic and highly sensitive chemical shift of 129Xe, and other noble gases has been widely used to probe the structure of molecules, microporous solids, such as zeolites and clathrates, and the surface features of membranes and other biological and non-biological materials. Recent improvements in the methods for producing hyperpolarized Noble gases have lead to many innovative NMR and MRI applications including medical imaging of the lungs and other parts of the body.
The technique typically used to produce hyperpolarized Noble gases involves the indirect transfer of angular momentum from optical photons to the nuclei of the noble gas molecules called “optical pumping and spin exchange.” Optical pumping uses an alkali metal intermediary such as Rb, K, or Cs with a valence electron carrying the spin polarization to polarize the Noble gas. An intermediary is used because the polarization of photons cannot be directly transferred to the nuclear spins of the Noble gas atoms.
In the conventional setting, an alkali metal such as rubidium is vaporized and mixed with a Noble gas. The mixture is irradiated with circularly polarized laser light at the wavelength of the first principal resonance (i.e. its principal electric-dipole transition). For rubidium, the wavelength is 795 nm, for example. The alkali metal vapor absorbs a photon and the valence electron transitions from a ground state to an excited state.
The total angular momentum of both the ground state and the excited state of the alkali metal is ½. Consequently, absorption of the circularly polarized light can only occur in the −½ ground state sublevel and not the +½ sublevel. Over time, essentially all of the Rb atoms are optically “pumped” into one sublevel because only one sublevel can absorb a photon. Under a modest magnetic field (10-80 Gauss), the cycling of alkali metal atoms between the ground and excited states can yield a substantial polarization of the atoms in a few microseconds. Thus, optical pumping creates electronic-spin polarization by selectively populating only one of the two possible spin states of the alkali-metal.
Exchange of the electronic orientation to the nuclear spin of the Noble gas takes place during binary collisions between the spin-polarized alkali metal atoms and the Noble gas atoms. During such collisions, the valence electron, through a hyperfine interaction, transfers angular momentum to the Noble gas nucleus causing a simultaneous nuclear and electronic spin flip. Thereafter, the alkali metal atom can absorb another photon and the process is repeated. In this manner, the nuclear polarization of the Noble gas can approach the level of the polarization of the irradiated alkali-metal vapor. Some production schemes provide a constant stream of hyperpolarized Noble gases that can be used in a magic angle spinning rotor or to circulate over or bubble through molecules in solution.
Hyperpolarized Noble gas atoms can also transfer spin polarization to the nuclei of atoms in sample molecules exposed to the gas. There are two primary techniques for the transfer of enhanced spin polarization from laser-polarized Noble gases to other nuclei such as protons that have been developed: (1) cross relaxation (SPINOE) and (2) cross polarization (CP).
The NMR signals of atoms in contact or close proximity to the hyperpolarized xenon are amplified due to the dipolar cross-relaxation and polarization transfer between the xenon and nuclear spins, a novel manifestation of the nuclear Overhauser effect termed SPINOE (spin-polarization-induced nuclear Overhauser effect). SPINOE also allows the transfer of spin polarization from laser-polarized gases to surface spins with no requirement for Hartman-Hahn matching of zero-field mixing. Solidification of the Noble gas is not required and consequently SPINOE can be carried out in a continuous flow mode and over a broader temperature range. Continuous flow of hyperpolarized Noble gas allows signal accumulation and therefore the exploration of surfaces with fewer spins or long relaxation times, as well as SPINOE under magic angle spinning.
Cross polarization, on the other hand, requires a static magnetic dipole interaction between the xenon spins and the nuclei that is the target of the transfer. With cross polarization, the xenon and the target nuclei are locked with simultaneous electromagnetic fields at two separate frequencies creating a quantum transition that allows the polarization to be transferred from the xenon to the target nucleus.
Hyperpolarized xenon and other noble gases can also be combined with a gas or fluid carrier that is chemically, biologically or materially compatible with the sample to be analyzed.
Poor sensitivity of conventional NMR detection coils at low fields has also been addressed with the use of superconducting quantum interference devices (SQUID), which have been used to obtain both spectra and imaging of laser polarized xenon for example. SQUID devices are presently one of the most sensitive detectors of magnetic flux. The AC or rf SQUID and the DC SQUID are the two main types of SQUID devices that have been developed. Generally, the SQUID device may be considered a flux to voltage converter consisting of a superconducting ring interrupted by one or more junctions called Josephson junctions and a large area flux antenna. Magnetic flux modulates the current passing through the Josephson junction.
However, SQUID devices exhibit instability in the presence of the pulsed magnetic fields that are necessary to prepare (encode) nuclear magnetization for detection. Consequently, these devices may be limited in their utility because of this instability.
Accordingly, there is a need for an apparatus and method for NMR/MRI spectroscopy that can optimize the encoding conditions and detection conditions without interference from ambient magnetic fields and overcomes many of the inherent limitations of traditional NMR devices. The present invention satisfies this need as well as others and generally overcomes the deficiencies of present devices and methods.